1. Field of the Invention
The present invention relates to an iris image definition estimation system using the astigmatism of the corneal reflection of a non-coaxial light source. More specifically, the present invention relates to a system using the astigmatism of the corneal reflection of a non-coaxial source to produce two virtual images of the light source on the meridional and sagittal image planes, and utilizes the shape of the composite virtual images to determine the resolution of an iris image and the direction of focus adjustment.
2. Description of the Prior Art
The iris is a complex structure in the eye and is a fibrous membrane consisting of an anterior layer of stroma and a posterior layer of epithelia cells. In general, the stroma layer has many unique features (e.g. crypts, collaret, and pupillary ruffs, etc.) around the pupil. The unique nature of the iris results in a set of binary features with more than 240 degrees of freedom which is notably higher than 80 degrees of freedom of the face and 20 to 40 degrees of freedom of fingerprints. Due to the fine texture of the iris, the probability of having the same iris texture is around 1 in 10 to the 78th power. Consequently, the patterns of the left and right iris of an individual may vary significantly. In addition, an iris pattern will not change as one ages. Thus far, iris recognition has achieved the highest recognition accuracy among all the commercialized biometric technologies and it is almost impossible to forge an iris pattern. The human iris is a tiny structure with a diameter around 1 cm. Hence, to obtain a high resolution iris pattern at a comfortable distance, a telephoto lens with a long focal length is necessary. However, the depth of the field of a telephoto lens is very shallow, and, therefore, only cameras with auto-focusing or guided focusing mechanism can produce a high resolution image for iris recognition. Both auto-focusing and guided focusing require a determination of the following factors: (a) a focus measure representing the resolution of the image and (b) the direction of the focus adjustment. In general, the focus measure of the image was determined by measurement of the intensity of the image high frequency components. Due to the extensive workload, a real-time calculation of the resolution cannot easily be achieved. Though Park and Kim used a near coaxial light source to significantly reduce the time for calculation, their method of iris focusing estimation technology cannot provide a focus adjustment direction from a single image when the iris pattern is close to the focus distance. Therefore, excess time has to be wasted in exploring the direction of focusing. Moreover, the unpredictable head movements of a user also contributes to multiple errors during the process of auto focus and can dramatically increase the time required for image acquisition.
A typical example for guided focusing iris imaging system is presented in U.S. Pat. No. 5,291,560A, example 1. This system is equipped with a camera, a semi-mirror and a small monitor. The semi-mirror is used to align the camera and the monitor. Images captured by the camera are shown on the monitor which provides feedback to aid the users in alignment and position adjustment of their heads so as to enhance the resolution of the iris pattern. Meanwhile, an image processing unit can automatically determine if the resolution of the iris image is acceptable for iris recognition. During the process, no assistance is available, thus, personnel training is necessary for image acquisition. Once the resolution of an image is qualified, the image undergoes a series of processes: iris localization, unwrapping, and feature extraction. Finally, the extracted feature is compared with the feature database for accurate identification.
Another system, US 2004/0101170A1 (example 2), utilizes the intensity of the high frequency signal to determine the resolution of the iris image. First, the system determines whether the outer boundary of the upper and lower eyelids can be found in the image or not. If the boundary is faint, then the image will be classified as blurry. Images passing the initial screening will be used for simple iris localization, and an region of interest (ROI) of the iris area around the pupil will be extracted. The sides of the iris have less interference by the eyelids and the eyelash, therefore, the gradient intensity of the image from this area can be used to estimate the resolution of the iris pattern. Nonetheless, the gradient intensity of the iris patterns varies from person to person, and as a result, no universal standards are available for a determination of whether the resolution of the iris pattern is qualified.
Another common system used to determine the resolution of an iris image by depth measurement is U.S. Pat. No. 6,714,665B1, example 3, which utilizes a pair of wide angle cameras to locate 3-D locations of the users' head and eye and to adjust the orientation of a flat mirror so as to allow another camera with a telephoto lens to capture the iris pattern of one of the user's eyes. During the eye-searching process of the controlling reflective mirror, the corneal reflection of two light sources was used to confirm the location of the eye. This system requires preliminary calibration and three cameras to locate the eye. The detailed information was obtained after calculation of the distance from the focus of the camera to the eye. In the process of tuning for sharp focus, a trial and error method was used. First, adjust the focus towards one direction, if the image is faint, re-adjust the focus towards the other direction, and vice versa. Such adjustments produce a sharp image. Nevertheless, this system is not cost-effective and is difficult to calibrate. Moreover, during the process of auto focus, slight movement of the user's head will significantly affect the time for adjustment and result in multiple errors.
An additional example is a portable iris recognition system using the intensity of the high frequency signal to determine the resolution of an iris image in U.S. Pat. No. 6,753,919B1 (example 4). For the purpose of auto focus, a concave cold mirror which reflects the visible lights while allowing the infrared light to pass is placed in front of the camera. The users can see the enlarged image of their eyes through the reflection of the concave mirror and the infrared image of the iris pattern is acquired by the camera. The image resolution is calculated after processing the image through a high-pass filter for evaluating the intensity of the high frequency components. An additional mirror can be added to this system and allows the users to see images of their eyes easily. However, this system is not equipped with a guided focusing system. Thus, the users have to speculate the distance in order to obtain a sharp image.
One example for determination of the resolution of the iris image is described in U.S. Pat. No. 7,095,901B2 (example 5). Extra lighting is added to the iris image acquisition system so as to accurately locate the position of a user's forehead and cheek for focusing calculation. This patent provides two possible methods: one is to estimate the distance using an extra camera, and the other is switching the iris photographing camera between two modes, distance measurement and image acquisition. The major problem of this method is that the distances between the camera and the user's head, and the iris pattern are not the same. Hence, the depth information of the forehead and/or the cheek cannot assure the resolution of an iris pattern.
Finally, a system using a near coaxial light to estimate the resolution of the iris image was reported by Park and Kim earlier (Kang Ryoung Park and Jaihie Kim,
“A Real-Time Focusing Algorithm for Iris Recognition Camera”, IEEE Transactions on Systems, Man, and Cybernetics, Vol. 35, No. 3, pp. 441-444, August 2005 (example 6). This system utilizes coaxial light as the incident light source, and the coaxial light will form a virtual image behind the cornea and near the iris after corneal reflection. If the focus of the camera is inaccurate, this virtual image will form a large glint area in a round shape due to defocus. Therefore, the size of the glint area can provide information of whether the iris image has high resolution or not. Since the intensity of the glint is inversely proportional to the distance between the eye and the camera, when the eye is away from the focused distance of the camera, the intensity of the glint can be used to discriminate the direction of focus adjustment. However, when the iris pattern is close to the focus distance, the techniques mentioned in example 6 can only estimate the resolution of an image, not the direction of focus adjustment. Accordingly, a trial-and-error method is also necessary for auto focus, and as mentioned above, slight movements of the user's head will significantly affect the time for adjustment and result in multiple errors.